The present invention relates to one-component, dual-curing conformal coating compositions.
Conformal coatings are used to protect circuit boards and electronic components from moisture and from premature corrosion. Polyurethane coatings are a popular choice for conformal coatings due to their high chemical resistance, flexibility, and hardness.
Coatings curable by actinic radiation (e.g. “UV coatings”) find use as conformal coatings as such coatings may be cured relatively quickly by exposure to a radiation source, typically UV light. The fast cure allows manufacturers to increase throughput. However, because circuit boards and other electronic components have highly contoured surfaces, such UV coatings, when used as conformal coatings, suffer from a phenomenon whereby areas of the coating which cannot easily be exposed to UV light due to the contours (i.e. the “shadow” areas) remain uncured.
U.S. Pat. No. 4,424,252 describes the problem of “shadow cure” and solves such problem by the use of dual cure resins. Such resins cure by two mechanisms: a) exposure to UV light and b) curing by atmospheric moisture. The atmospheric moisture reacts with free isocyanate groups in the dual cure resin forming an amine group(s). The amine groups react with other isocyanate groups to form a polyurea. The curing by atmospheric moisture allows the areas of the coatings in the “shadows” to fully cure.
However, dual-cure polyurethane resins have suffered a disadvantage due to their relatively high viscosity. This issue is especially relevant for conformal coatings, due to the contoured shape of the substrate. Low viscosity resins are more favoured from a flowability standpoint—i.e. the low viscosity resins more easily flow around the contours of a circuit board and may more evenly coat such circuit boards.
Previous attempts to lower the viscosity of dual-cure polyurethane resins included adding a significant amount of reactive diluent to the dual-cure polyurethane resins. While the addition of reactive diluents served to lower the viscosity of the resin, it also reduced the wt. % of free isocyanate groups in the resin. This, in turn, has the effect of reducing the effectiveness of the moisture cure mechanism especially in shadow areas, which results in a coating with insufficient hardness and/or solvent resistance in such areas.
An object of the present invention is to provide coating compositions, which are suitable as conformal coatings and are characterized by their ability to be cured by exposure to actinic radiation, their ability to be cured by exposure to atmospheric moisture and their relatively low viscosity and that permit to obtain coatings with sufficient hardness and solvent resistance, especially in shadow areas
This object may be obtained with the coating compositions according to the invention, which are described below in greater detail.